Locales support about duckling

[patapizza]

On another note, there are languages not included in the two-letters ISO-639-1 standard, but have a ISO-639-3 representation (e.g. Filipino).
Some languages that have an ISO-639-1 representation are considered as macrolanguages in ISO-639-3 (e.g. Chinese). Cantonese has no ISO-639-1 representation so we've included Cantonese variations for Time within the Chinese rules. But Cantonese has a ISO-639-3 representation (yue).

From a locale perspective, as Cantonese is an official language in Hongkong and Macau, it is represented as zh-HK and zh-MO.
Locales capture regional particularities of a language. For example, in French the numeral 70 is pronounced "soixante-dix" in France (fr-FR), and "septante" in Belgium (fr-BE) and Switzerland (fr-CH).

To summarize, it seems like we could add ISO-639-3 languages if they don't derive from another ISO-639-1 (macro)language (e.g. fil for Filipino), and support locales as language extensions (e.g. zh-HK/zh-MO for Cantonese, fr-BE for French in Belgium).

Edit: It seems like Tagalog relates to Filipino the same way Castilian relates to Spanish. Tagalog has a ISO-639-1 representation (tg). So let's keep using ISO-639-1 language codes with locales extensions.